1. Field of the Invention
The present invention relates to an accelerometer and, more particularly, to a bonding method for reducing mechanical stress at the magnet interface of a force rebalance accelerometer which includes a proof mass suspended between one or more magnet assemblies.
2. Description of the Prior Art
Force rebalance accelerometers which include a proof mass suspended between one or more magnet assemblies are generally known in the art. Examples of such accelerometers are disclosed in U.S. Pat. Nos. 4,182,187; 4,250,757; 4,394,405; 4,399,700; 4,400,979; 4,441,366; 4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,228; 4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172; 5,111,694; 5,182,949; 5,203,210; 5,212,984; and 5,220,831, all herein incorporated by reference. Such force rebalance accelerometers normally include a proof mass, known to be formed from amorphous quartz, suspended by one or more flexures to enable the proof mass to deflect in response to forces or accelerations along a sensitive axis, generally perpendicular to the plane of the proof mass. At rest, the proof mass is normally suspended equidistantly between upper and lower excitation rings. Electrically conductive material forming pick-off capacitance plates is disposed on opposing sides of the proof mass to form capacitive elements with the excitation rings. An acceleration or force applied along the sensitive axis causes the proof mass to deflect either upwardly or downwardly, which causes the distance between the pick-off capacitance plates and the upper and lower excitation rings to vary. This change in the distance between the pick-off capacitance plates and the upper and lower excitation rings causes a change in the capacitance of the capacitive elements. The difference in the capacitances of the capacitive elements is thus representative of the displacement of the proof mass along the sensitive axis. This displacement signal is applied to a servo system that includes one or more electromagnets which function to return the proof mass to its null or at-rest position. The magnitude of the drive currents applied to the electromagnets, in turn, is representative of the acceleration or force along the sensitive axis.
The electromagnets are known to include a magnet formed from, for example, alnico, normally bonded to an excitation ring formed from a material having relatively high permeability, such as Invar, to form a magnetic return path. The materials used for the magnet and the excitation ring will have different coefficients of thermal expansion, since the materials are different. As such, the interface defined between the magnet and the excitation ring will be subject to stress as a function of temperature. Such stress over a period of time and/or temperature degrades the performance of the accelerometer.
In order to resolve this problem, compliant epoxies have been used to bond the magnet to the excitation ring. However, such compliant epoxies degrade the long term stability of the accelerometer.